Utilization of biomass has received a lot of attention for the development of a sustainable society. Manufacture of organic commodity chemicals from biomass is one of the most important methods of future biomass utilization because biomass is the only renewable source of carbon. Lignocelluloses biomass acts as a more promising feedstock for downstream applications which is more abundant, cheaper and potentially more sustainable.
Furfural is typically used as a precursor in the production of tetrahydrofuran (THF), an important industrial solvent.
Furfural is obtained from the dehydration of pentoses, five carbon sugars, such as xylose and arabinose, commonly obtained by acid-catalyzed digestion of hemicellulose-rich agricultural wastes. The acid hydrolysis of pentose sugars followed by dehydration of three water molecules gives furfural (C5). Furfural (FFR) deserves an attention as a potential platform molecule for production of variety of value added chemicals and fuels such as furan, furfuryl alcohol (FA), tetrahydrofurfuryl alcohol (THFA), 2-methyl furan (2-MF) tetrahydrofuran and 2-methyl tetrahydrofuran (2-MTHF), in which tetrahydrofuran can be produced by decarbonylation of furfural to furan under reductive conditions and then subsequently hydrogenated to tetrahydrofuran (THF). It possesses wide applications; it can be used as solvent, monomer for polymer and chemical.
However depending upon reaction conditions and catalytic properties, hydrogenation of FFR gives variety of products, all of which are important intermediates in the chemical industry.
Conventionally, THF can be produced by number of different routes including dehydration of 1,4-butanediol (BDO) and hydrogenation of maleic anhydride, all these processes contains multi-step synthesis by using reagents and metal complexes for production of THF, out of these one is starting from acetylene and formaldehyde in the presence of a cuprous acetylene complex to form butynediol (Reppe Process), while these processes were dependent on petroleum feedstock.
These processes suffer from a variety of deficiencies such as multi-step synthesis, heavy metal complexes, reagents and harsh reaction conditions which is directly relevant to environmental hazardous.
Hence, these drawbacks may be overcome by either novel catalytic system and/or a single step process for synthesis of THF from furfural which bio-renewable.
There are many supported noble (Pt, Pd, Ru, Rh) and non-noble (Cu, Ni,) metals that have been reported for the hydrogenation of FFR. Mahajani et al in Ind. Eng. Chem. Res., 2003, 42, 3881-3885 reported 5% Pt/C catalyst to study kinetics of FFR hydrogenation to FA, the order he found was 0.85 at 403-448 K temperature and 1.03-2.06 MPa H2 pressure.
Baijun et al in Applied Catalysis A: General, 1998, 171, 117-122 reported Raney nickel catalyst for furfural hydrogenation to furfuryl alcohol with 98% selectivity.
Twin catalyst system for the continuous conversion of furfural to THF through intermediate furan in supercritical Co2 with high selectivity to THF is reported by M. Poliokoff et. al. in Angew. Chem. Int. Ed., 2010, 49, 8856-8859.
Guha J. Catal., 1985, 91, 254-262 reported vapour phase decarbonylation of furfural over Pd—Al2O3 catalyst, deactivation of catalyst found by cocking Pd—Al2O3 catalyst. Resasco in Catal Lett, 2011, 141, 784-791 studied the different silica supported metal catalyst for the hydrogenation of furfural, he proposed 1% Pd/SiO2 catalyst give 20% THF with 69% conversion in a continuous fixed bed reactor.
Nagaraja et al. in J. Mol. Catal. A: Chemical, 2007, 278, 29-37 reported Cu based catalyst for coupling route highlighting the combination FFR hydrogenation and dehydrogenation of cyclohexanol in vapour phase conditions. Vapour phase hydrogenation of FFR studied over Ni/SiO2 catalyst to THFAL via furfuryl alcohol in two step strategy having no selectivity found to be THF.
Recently Rafael et al. in Green Chem., 2012, 14, 1434-1439 reported hydrogenation of furfural using carbon-supported Pd NPs with 90% conversion of FFR and 80% selectivity to THF on micro reactor. In “The electrocatalytic hydrogenation of furanic compounds in a continuous electrocatalytic membrane reactor” by Sara K. Green, Jechan Lee, Hyung Ju Kim, Geoffrey A. Tompsett, Won Bae Kim and George W. Huber in Green Chem., 2013, 15, 1869-1879, demonstrate the use of a continuous-flow electrocatalytic membrane reactor for the reduction of aqueous solutions of furfural into furfuryl alcohol (FA), tetrahydrofurfuryl alcohol (THFA), 2-methylfuran (MF) and 2-methyltetrahydrofuran (MTHF). Protons needed for hydrogenation were obtained from the electrolysis of water at the anode of the reactor. Pd was identified as the most active monometallic catalyst of 5 different catalysts tested for the hydrogenation of aqueous furfural with hydrogen gas in a high-throughput reactor. Thus Pd/C was tested as a cathode catalyst for the electrocatalytic hydrogenation of furfural. At a power input of 0.1 W, Pd/C was 4.4 times more active (per active metal site) as a cathode catalyst in the electrocatalytic hydrogenation of furfural than Pt/C. The main products for the electrocatalytic hydrogenation of furfural were FA (54-100% selectivity) and THFA (0-26% selectivity). MF and MTHF were also detected in selectivities of 8%. Varying the reactor temperature between 30° C. and 70° C. had a minimal effect on reaction rate for furfural conversion. Using hydrogen gas at the anode, in place of water electrolysis, produced slightly higher rates of product formation at a lower power input. Sparging hydrogen gas on the cathode had no effect on reaction rate or selectivity, and was used to examine the addition of recycling loops to the continuous electrocatalytic membrane reactor.
A cursory review of prior arts reveal that there is still a scope in the art to provide an effective catalytic system that provides conversion rates with higher selectivity towards tetrahydrofuran.